Abstract

We study the dynamics of energy deposition and subsequent material response associated with exit surface damage growth in fused silica using a time resolved microscope system. This system enables acquisition of two transient images per damage event with temporal resolution of 180 ps and spatial resolution on the order of 1 µm. The experimental results address important issues in laser damage growth that include: a) the specific structural features within a damage site where plasma formation initiates; b) the subsequent growth of the plasma regions; c) the formation and expansion of radial and circumferential cracks; d) the kinetics and duration of material ejection; e) the characteristics of the generated shockwave.

Images of a typical damage growth event in the transmission geometry capturing (a) the initial, the transient at (b) −3.81 ns and (c) −1.16 ns delays, and (d) the final damage site morphologies. The corresponding normalized images are shown in (a1), (b1), (c1) and (d1), respectively. Several plasma initiation sites in the transient images are indicated by arrows. The green outline in (b1) separates the inner “core” region from the outer region containing mostly cleaved and/or cracked surfaces. The spatial scale applies to all images.

Images of a typical damage growth event in the transmission geometry capturing (a) the initial, the transient (normalized) at (b) −0.44 ns and (c) + 2.21 ns delays, and (d) the final (d1: normalized) damage site morphologies. The green outline in (a) outlines the inner “core” region from the outer cleaved and/or cracked surfaces. The spatial scale applies to all images.

Images of a typical damage growth event in the transmission geometry capturing (a) the initial, the transient (normalized) at (b) + 21.09 ns and (c) + 23.74 ns delays, and (d) the final damage site morphologies. Arrows 1 and 2 indicate interesting transient features: radial cracks formed after laser energy deposition and the air shockwave, respectively. The spatial scale applies to all images.

The radial speed of expansion of plasma sites formed at the DSP interface as a function of (a) the delay of the probe-1 (solid black circles) and (b) the plasma radius as captured by the probe-1 transient image (solid green circles). Trends in the data are shown by the red solid line in (a) and black solid squares in (b)–average values upon data binning.

(a) Estimates of particle speeds and sizes at various delay times and (b) plot of the ratio of average speed Vave to estimated speed Vest vs. delay time for exit surface damage growth in fused silica under excitation with 355 nm, 8 ns laser pulses with average fluence of ~18 J/cm2.

The distance of the air shockwave (solid circles) from the surface as a function of the delay time for damage initiation and subsequent damage growth. The blue line profiles represent qualitative fits to the upper (solid line) and lower (dashed lines) boundaries of the experimental data and yield the upper and lower bounds on the speed of the shockwave (solid and dashed red lines, respectively).